La Niña's Ghost

La Niña has faded away, but will weather patterns
change? Some scientists expect the Pacific Decadal Oscillation
to pick up where La Niña left off.

September 15, 2000 -- The El Niño/La Niña
one-two punch that caused unusual weather to hit much of North
America over the last three years has finally come to an end,
a NASA satellite image shows -- leaving behind "bruises"
in the form of charred forests and an above-average hurricane
season.

So the big question is: with La Niña gone, will the
weather return to normal this winter? Well ... maybe.

Some scientists say weather patterns are likely to return
to normal, while others point to signs of another ocean temperature
pattern in the Pacific called the Pacific Decadal
Oscillation (PDO), which could perpetuate La Niña-like
conditions.

Above: Image from the NASA/CNES
TOPEX/Poseidon
satellite showing the return of normal ocean temperatures
(green) to the tropical Pacific. Note also the above average
temperatures (yellow, red and white) in the north, west and south
Pacific, which may reveal a lingering negative phase of the Pacific
Decadal Oscillation (PDO). Image courtesy of JPL.

"Anybody who makes a hard and fast forecast for this
winter has definitely got a better Ouija board than me,"
said Dr. William Patzert, an oceanographer with NASA's Jet Propulsion
Laboratory in Pasadena, CA.

One source of ambiguity in the winter outlook is a disagreement
among scientists about the existence and potential impacts of
a "negative phase" of the PDO.

The PDO is a long-term cycle observed in records of sea surface
temperature for the Pacific Ocean. In the "negative"
half of the cycle, cooler sea surface temperatures in the east
and center of the Pacific are surrounded by warmer temperatures
in the north, west and south that form a "horseshoe"
shape -- a pattern similar to La Niña, only much bigger.

"In many ways when the PDO is in its cool (negative)
phase, it looks like a larger La Niña ," Patzert
said. "El Niño and La Niña are confined
to the tropics, but PDO takes up the whole Pacific. And
it has longer time scales."

Each phase of the PDO lasts about 10 to 20 years, according
to Patzert. The last negative phase spanned roughly 1945
to 1970, followed by a positive phase lasting roughly from 1975
to 1995. The "positive" phase of the cycle has
a similar shape, but with the temperatures reversed.

Potentially, those sea surface temperature patterns could
divert the North American jet stream northward into Canada, producing
weather effects reminiscent of La Niña.

"When you have the negative phase of the PDO ... the
general tendency is to keep the jet stream a little further north,
in contrast with El Niño, which dropped it so far south,"
Patzert said. "These tendencies look a lot like La
Niña."

For the winter, that would mean colder than normal temperatures
in the northwestern half of North America and warmer than normal
temperatures in the Southeast. The Pacific Northwest and
the Mississippi and Ohio river valleys would receive more precipitation
than normal, while California and the Eastern Seaboard states
south of Maryland would receive less.

"Everything
with regard to weather -- temperature, rainfall -- has to do
with the path of the jet stream," Patzert said.

However, Patzert cautioned, the relationship between the PDO
and changes in the path of the jet stream is a hot area of scientific
research and still involves a certain amount of speculation.

"Actually, I'm not even making a forecast; I'm just saying
there are tendencies," Patzert said. "This whole
PDO thing, you have to remember, is new territory. Everybody
is hedging."

While temperature records indicate that the PDO has been going
on for a long time, the scientific community only became aware
of the pattern in 1996, when Steven Hare and his colleagues at
the University of Washington discovered the cycle while researching
fish population fluctuations.

What are the implications of the PDO for North American climate
in the coming season? "People are scratching their heads,
because the PDO is a relatively new phenomenon," Patzert
said. "In contrast, the El Niño and the La Niña
have a very strong signal, and we've been looking at them for
30 years."

PDO research is so new that scientists aren't yet sure that
the cycle is indeed in its negative phase.

Dr. Vernon Kousky, a research meteorologist with the National
Oceanic and Atmospheric Administration's Climate Prediction Center,
uses a different index to characterize the PDO, and his conclusions
differ from Patzert's.

"Our indices indicate that PDO is pretty much neutral,"
Kousky said. "I looked at the ocean temperature anomalies
just recently for the North Pacific and there's still some residual
... negative PDO indications up there," Kousky said, "but
the pattern is broken up and does not look as coherent as it
did, say, about six months ago or so.

Above: A recent daily map of
sea surface temperature anomalies produced by Al Strong of NOAA.
The orange horseshoe shape
in the Pacific is characteristic of a negative PDO phase.

"We expect to see a much more normal-ish (weather) pattern
developing over the next couple months and into the winter,"
Kousky said.

Ironically, even a "normal" winter would entail
greater uncertainty in the seasonal forecast for specific regions
than during these last three years of El Niño and La Niña.

"The atmosphere can do a number of things on its own
just because of its natural behavior," Kousky continued. "And
when you take away the primary forcing -- which would be, say,
large scale temperature patterns in the tropics with the El Niño
and La Niña -- then the atmosphere's free to pretty much
do what it can do, and it has a wide range of possibilities.

"Even though our forecast says we expect most areas to
be leaning toward warmer than normal, it wouldn't surprise us
at all to see some colder than normal temperatures somewhere
in the U.S.," Kousky said.

This natural variation in the atmosphere's behavior in the
absence of a strong El Niño or La Niña may help
explain why this summer's weather patterns have differed from
those expected for a La Niña year.

"I think we saw a gradual breakdown in the La Niña
pattern as the summer progressed," Kousky said. "Since
about May or so, the traditional La Niña conditions have
faded away and kind of shifted into a different pattern.

"(It) probably represents just part of the natural variability
of the atmosphere," Kousky said.

In fact, the waning influence of La Niña on the summer's
weather patterns raises the question of whether this summer's
wildfires can be blamed solely on La Niña.

"The
pattern of drought we had (over the summer) cannot be attributed
to La Niña," said Dr. James O'Brien, director of
the Center for Ocean-Atmospheric Prediction Studies at Florida
State University in Tallahassee, FL.

O'Brien noted that droughts in the Montana-Idaho region and
in Georgia are not typical for a La Niña summer.

"Something else is going on," O'Brien said. "Unfortunately
with El Niño and La Niña, many things are blamed
on it without people doing the statistics."

Above: This August 2000 image shows the latest normalized
difference vegetation index (NDVI,
which indicates plant vigor) anomaly data for North America.
The area of stressed vegetation in the Rocky Mountains has intensified
and spread compared to July 2000. It is likely that smoke from
the fires that burned throughout August caused the extremely
low values of NDVI anomaly shown in Idaho and Montana. [more
information]

Still, La Niña probably did set the stage for the fires.

"I think (the conditions leading to the wildfires) have
to be considered as probably a residual La Niña effect,"
Kousky said. "To have a spring or early summer warmer
than normal out there (western U.S.), that is a La Niña
signal. And to have it be somewhat on the dry side is also
a La Niña signal."

While the firefighters battling the blazes out west might
wish that La Niña was gone for good, it's only a matter
of time before it returns. Scientists can't predict exactly
when a La Niña or El Niño event will occur, but
El Niño won't reappear until next year at the earliest,
and La Niña is likely to repeat between four and seven
years from now.